1. Field of the Invention
The present invention relates to a reflective liquid crystal display (LCD) apparatus having a reflection plate for reflecting to outside a light which has passed through a liquid crystal layer from outside.
2. Description of the Related Art
As compared to a transmitting LCD apparatus, a reflective LCD apparatus can reach a reduced power consumption and a thin type with a reduced weight and accordingly, has been used mainly in a portable terminal. In the reflective LCD apparatus, light incident from outside is reflected by a reflection plate in the apparatus so as to be utilized as a display light source, thereby eliminating need of back light.
The conventional reflective LCD apparatus has a basic configuration composed of a liquid crystal of the TN (twisted nematic) type, a single deflection plate type, STN (super twisted nematic) type, GH (guest-host) type, PDLC (high molecule dispersion) type, or cholesteric type, a switching element for driving the liquid crystal, and a reflection plate arranged inside or outside liquid crystal cells. The reflective LCD apparatus utilizes an active matrix drive method capable of realizing a high-resolution and high-quality image by using a thin film transistor (TFT) or metal/insulation film/metal structured diode (MIM) as a switching element, to which a reflection plate is attached.
FIG. 36 is a cross sectional view of a conventional reflective LCD apparatus of a single deflection plate type. Hereinafter, explanation will be given with reference to this figure.
An opposing substrate 1 includes a deflection plate, a phase difference plate 32, a glass substrate 4, a color filter 5, and a transparent electrode 6. A lower substrate 7 includes a glass substrate 8, a thin film transistor (TFT) 9 of an inverse stagger structure as a switching element formed on the glass substrate 8, an insulator protrusion 10 as a base for forming a, a polyimide film 11 as an inter-layer insulation film formed thereon, and a reflection electrode 13 connected to a source electrode 12 of the TFT and functioning as a reflection plate and a pixel electrode. A liquid crystal layer 14 is arranged between the opposing substrate 1 and the lower substrate 7.
A reflected light 16 is utilized as a light source. An incident light 15 from outside passes through the deflection plate 2, the phase difference plate 3, the glass substrate 4, the color filter 5, the transparent electrode 6, and the liquid crystal layer 14, and is reflected by the reflection electrode 13 to become the reflected light 16.
This reflective LCD display apparatus should have a display performance for displaying a bright and white display when the liquid crystal is in the light transmitting state. In order to realize this display performance, it is necessary to effectively eject forward the incident light 15 from various directions. For this, the polyimide film 11 is formed with the convex/concave structure, so that the reflection electrode arranged thereon can have a scattering function. Accordingly, control of the convex/concave structure of the reflection electrode 13 is the important factor in deciding the display performance of the reflective LCD apparatus.
FIG. 37 and FIG. 38 show a production method of a conventional reflective LCD apparatus in cross sectional views. Hereinafter, explanation will be given with reference to these figures.
In the thin film transistor production procedure, firstly, a gate electrode 21 is formed on the glass substrate 20 (FIG. 37[a]). Next, a gate insulation film 22, a semiconductor layer 23, a doping layer 24 are formed (FIG. 37[b]). Next, an island 25 of the semiconductor layer 23 and the doping layer 24 is formed (FIG. 37[c]), and the source electrode 26 and the drawing electrode 27 are formed (FIG. 37[d]). After this, the reflection electrode is formed.
For forming the reflection electrode, firstly, an organic insulation film 28 having photosensitivity is formed (FIG. 37[e]). Then, photolithography is performed to form a protrusion 29 in the reflection electrode forming region (FIG. 37[f]), which is then melted by heating so as to be formed into a smooth protrusion 30 (FIG. 38[g]). Next, the protrusion is covered by an organic insulation film 31 to obtain a further smooth convex/concave surface 32 (FIG. 38[h]). Next, a contact portion 33 is formed for electrically connecting a reflection electrode to a source electrode of the thin film transistor (FIG. 38[i]), and then the reflection electrode 34 is formed (FIG. 38[j]). The method for forming this reflection electrode is disclosed, for example, in Japanese Patent Publication (examined) 61-6390 or in Tohru Koizumi and Tatsuo Uchida, Proceeding of the SID, Vol. 29, 157, 1988.
As has been described above, in the conventional reflective LCD apparatus, the convex/concave structure is formed by organic insulation film or inorganic insulation film having photosensitivity as a base which is covered by an organic insulation film or inorganic insulation film.
However, below the protrusions, there are formed a metal wiring, an electrode, a switching element, and the like, which are exposed to an etching liquid used in the etching procedure for forming the protrusions. As a result, a reaction between the etching liquid and the undercoat film deteriorates characteristic of the switching element and the remaining etching liquid lowers reliability of the switching element.
Moreover, when using an organic insulation film or inorganic insulation film having no photosensitivity for the insulation film below the reflection electrode, a photo resist pattern is formed on the insulation film and dry etching is performed to form a convex pattern. In this case, the undercoat film is exposed to plasma during the etching and the plasma damage deteriorates the characteristic of the switching element.
On the other hand, the conventional method for producing the conventional reflective LCD apparatus requires a number of production steps as has been described above. This increases the production cost, which in turn increases the cost of a reflective LCD apparatus. The reason why the reflective LCD apparatus requires a number of production steps is that a high-performance switching element and a high-performance reflection plate are formed on the same insulation substrate in order to obtain a bright high-quality display, and that the production of the high-performance reflection plate requires a method capable of forming the convex/concave structure on the reflection plate surface with a desired configuration. Accordingly, the conventional reflective LCD apparatus requires a number of film formation steps, photoresist (PR) steps, and etching steps.
Currently, no effective means is employed to simplify the production procedure. The convex/concave structure below the reflection electrode is currently produced as follows. Firstly, a photosensitive resin is applied, which is then patterned by an exposure step and a development step so as to form a convex pattern. However, in the area other than the portion having this convex pattern, the photosensitive resin film is completely removed. After this, the convex pattern is subjected to a thermal treatment so as to obtain a smooth protrusion shape, which is then covered by an organic insulation layer so as to obtain a desired smooth convex/concave surface.
That is, the insulation film below the reflection electrode consists of two layers: a film of convex shape and a film covering it. This insulation film has a function as an inter-layer insulation film for electrically insulating the reflection electrode from the switching element and the wiring. After this, a contact hole is formed in this insulation layer. Then, a metal thin film such as aluminum is layered thereon. This metal thin film is patterned to obtain a reflection electrode along the fine convex/concave structure of the insulation film.
Thus, formation of the reflection electrode has required five steps: (1) formation of an insulation film for forming a protrusion as a base; (2) formation of a protrusion; (3) formation of a contact hole; (4) formation of a metal thin film having a high reflection efficiency; and (5) formation of a reflection electrode.
It is therefore an object of the present invention to provide a reflective LCD apparatus and a production method thereof for enabling a high-luminance and high-quality display capability by preventing deterioration of the switching element in the production procedure as well as reducing the production cost by reducing the number of production steps.
The reflective liquid crystal display (LCD) apparatus according to the present invention includes: a transparent first substrate; a transparent electrode arranged on the first substrate; a second substrate; a switching element arranged on the second substrate; an insulation film arranged on the switching element and having a convex/concave structure; a reflection electrode arranged on the insulation film along the convex/concave structure and connected to the switching element; and a liquid crystal layer sandwiched between transparent electrode of the first substrate and the reflection electrode of the second substrate. The insulation film protects the switching element after formed and the convex/concave structure is formed by irregular arrangement of regions having different thickness values.
In a conventional reflective LCD apparatus, protrusions are located on a metal wiring, electrode, switching element, and the like, which are exposed to a process atmosphere when forming the protrusions, causing deterioration of the switching element. According to the present invention, since the insulation film always covers the metal wiring, the electrode, the switching element, and the like, they are not exposed to a process atmosphere can be protected from a process damage. Moreover, in the present invention, the insulation film has regions having different film thickness values, i.e., protrusions having a large film thickness and indentations having a small film thickness. This eliminates need to form another film for the convex/concave structure.
According to another aspect of the present invention, the convex/concave structure has a continuous smooth shape. This enables to obtain a bright display because the luminance of the reflective LCD apparatus is determined by the inclination angle of the convex/concave structure of the reflection electrode.
According to yet another aspect of the present invention, the insulation film may be a single-layered film made from a single material. Thus, the insulation film is formed by a single layer by a single step. That is, there is no need to form the convex/concave structure and the inter-layer insulation portion by separate steps. This simplifies the convex/concave structure formation step which is complicated in the conventional reflective LCD apparatus.
According to still another aspect of the present invention, the insulation film may have a light absorption characteristic. Thus, the insulation film can absorb an incident light from between adjacent reflection electrodes. This enables to shut out the incident light which may be introduced to the back side of the reflection electrode, thereby suppressing radiation of the incident light to the switching element and enabling to realize a preferable switching characteristic.
According to still yet another aspect of the present invention, the convex/concave structure may have a plurality of protrusions arranged irregularly. This can suppress interference of the reflected light from the reflection electrode, thereby enabling to form a convex/concave structure having a preferable reflection capability. Furthermore, the protrusions may have an island shape or a line shape in a plan view. This enables to obtain a bright reflection capability. That is, in the reflective LCD apparatus using such a convex/concave structure, it is possible to obtain a bright display characteristic.
Moreover, the convex/concave structure may have a plurality of indentations arranged irregularly. This suppresses interference of the reflected light from the reflection electrode, thereby enabling to form a convex/concave structure having a preferable reflection characteristic. Furthermore, the indentations may have a hole shape or a line shape in a plan view. This enables to obtain a bright display characteristic. That is, in the reflective LCD apparatus using such a convex/concave structure, it is possible to obtain a bright display characteristic.
Moreover, the convex/concave structure may be formed by repetition of an irregular convex/concave shape based on one or more than one pixels. This can suppress interference of the reflected light. Accordingly, the reflective LCD apparatus produced by using this reflection electrode has no wavelength dependency by the light source or no deterioration of the color characteristic, thereby enabling to obtain a bright high-quality display characteristic.
Moreover, the insulation film having the convex/concave structure may be made from an organic resin or inorganic resin having photosensitivity. In this case, it is possible to form a desired convex/concave pattern by performing exposure and development directly to the photosensitive resin, eliminating the need of photoresist application, formation, development, and peel-off steps. Thus, the number of production steps is reduced, thereby enabling to reduce the cost of the reflective LCD apparatus.
The present invention also provides a reflective LCD apparatus production method for producing the reflective LCD apparatus. That is, the convex/concave structure is formed by performing photolithography to the insulation film to form a predetermined pattern while leaving a predetermined film thickness, so as to form regions having a large film thickness and regions having a small film thickness arranged irregularly in a plan view.
Thus, the convex/concave structure is formed in the insulation film using a mask pattern, which enables to accurately control a plan shape of the convex/concave pattern and to form a desired convex/concave pattern with a high reproducibility. Furthermore, when the insulation film is etched leaving a desired film thickness, it is possible to control the cross sectional shape of the convex/concave pattern with a high reproducibility. Accordingly, it is possible to realize a preferable convex/concave structure. Moreover, this can be performed by a single photoresist step and a single etching step. This can significantly simplify the production procedure. Moreover, since the metal wiring, the electrode, the switching element, the insulation film are not exposed to the process atmosphere (etching liquid, etching gas, and the like) and are not damaged, thereby enabling to realize a reflective LCD apparatus having a preferable element characteristic.
Furthermore, according another aspect of the present invention, the convex/concave structure may be formed by steps of: forming the insulation film, photolithography for forming a resist pattern on the insulation film, etching the insulation film leaving a predetermined film thickness at a lower portion of the insulation film, peeling off the resist film from the insulation film, and thermal treatment of the etched insulation film to melt the insulation film and make the convex/concave structure smooth.
According to this production method, it is possible to form a convex/concave pattern without exposing thee switching element, the wiring, the electrode, and the like located under the insulation film. Accordingly, it is possible to form the convex/concave pattern without damaging the switching element and the like. Moreover, the convex/concave insulation film under the reflection electrode, unlike the convex/concave insulation film in the conventional reflective LCD apparatus, does not need a step for forming basic protrusions and a step for forming a film thereon. The convex/concave insulation film can be formed by using a single film and by a single step. This simplifies the production procedure.
Furthermore, the convex/concave structure may be formed by steps of: forming the insulation layer using an organic insulation material or inorganic insulation material having photosensitivity, performing exposure for forming a convex/concave pattern on the insulation layer, development for performing etching-development so as to leave a predetermined film thickness at a lower portion of the insulation film, and performing thermal treatment of the etched and developed insulation film to melt the insulation film and make the convex/concave structure smooth.
This eliminates the resist application, formation, development, and peel-off steps required for forming the convex/concave structure. Exposure and development can be performed directly to the photosensitive resin to obtain a desired convex/concave pattern. Thus, the production procedure is further simplified, thereby reducing the cost for producing the reflective LCD apparatus.
Moreover, it is possible to use an organic insulation material or inorganic insulation material having photosensitivity for the insulation film, to which the convex/concave structure and the contact hole are simultaneously formed by a single development step. This enables to form the convex/concave structure and the contact hole without using the resist process.
Here, the photosensitivity may be positive type, and the step of exposure may be performed in such a manner that a smaller exposure light quantity is applied for formation of the convex/concave pattern and a greater exposure light quantity is applied for formation of the contact hole pattern. This eliminates the contact formation step, thereby simplifying the production procedure.